Flexural Performance–Driven Optimization of Steel Slag as a Partial Cement Substitute in Paving-Grade Concrete for Sustainable Rigid Pavements

Abstract

Rigid pavement thickness design is governed by flexural strength, yet studies of steel slag as a cement replacement have overwhelmingly reported compressive strength alone. Paving-grade concrete in which ordinary Portland cement was replaced by ground steel slag at 0, 20, 30, 40, and 50% by mass was tested for slump and for cube compressive and third-point flexural strength at 7 and 28 days. The response was not a smooth optimum but a plateau terminated by an abrupt threshold. Up to 30% replacement, 28-day compressive (54.11–54.35 MPa) and flexural (6.52–6.61 MPa) strengths were indistinguishable from the control. Between 30 and 40%, compressive strength fell by 23.6% and flexural strength by 22.5%, taking both below the 48.25 and 5.16 MPa target mean strengths required for acceptance. The flexural-to-compressive ratio, constant at 0.891 MPa up to 30%, fell to 0.760 at 50%, indicating a disproportionate loss of tensile capacity. The 30% mix satisfied all acceptance criteria, cut cement demand by 89 kg/m³ (22.8%) and materials cost by 1.7%, but required double the admixture dosage.